1. Field of the Invention
The present invention relates to a double diaphragm-type semiconductor pressure sensor adopting a double diaphragm structure to ensure that a fluid whose pressure is to be measured does not come into direct contact with a semiconductor diaphragm on Which a distortion gauge is formed.
2. Description of the Conventional Art
In cases where a semiconductor pressure sensor is used for measurement of pressure of a liquid, such as water pressure and oil pressure, or a mixture thereof, a double diaphragm structure in which a metallic sealed diaphragm is attached to a pressure introducing portion is generally used to avoid the deterioration of the characteristic through the adherence of sediments of the liquid on a semiconductor diaphragm on which a distortion gage is formed. An example of a semiconductor pressure sensor to which this double diaphragm structure is applied is shown in FIG. 2A. In a silicon chip 1, a diaphragm is formed in a central portion thereof by means of etching, and four distortion gages are formed on the diaphragm in a dispersed manner and constitute a bridge circuit. This silicon chip is secured to a container 3 via a pedestal 2 formed of silicon or glass for alleviating a thermal stress. As shown in FIG. 2B, a space 100 surrounded by the pedestal 2 and a recessed portion located in a surface of the diaphragm of the silicon chip 1 is sealed into a vacuum. An upper opening of the container 3 is closed by a sealed diaphragm 4 which is made of stainless steel and whose periphery is airtightly coupled with the container 3, and an insulating oil 5 such as silicone oil is sealed in the interior of a space including the silicon chip 1 and the pedestal 2. A cover 6 airtightly coupled with the periphery of the container 3 is opposed to a top of the sealed diaphragm 4, and a space surrounded by the cover 6 and the sealed diaphragm 4 communicates with the outside via a hole 7 in the cover 6. Input/output voltages to and from the distortion gage bridge are led to outside the container 3 via wires 8 and leads 9 and are connected to the wires of a circuit board 40. An operational amplifier chip 41, a thick-film resistor 42, and the like are mounted on the circuit board 40, thereby constituting a signal processing circuit for amplifying a gage bridge output signal and effecting the compensation of the temperature and adjustment of the characteristic. An output from this circuit is led out to the outside through a lead 10. A major portion of the container 3 and the circuit board 40 are covered with a molded resin 43 for protection.
Pressure to be measured passes through the hole 7 of the cover 6 and is imparted to the sealed diaphragm 4, and is further transmitted to the diaphragm on the silicon chip 1 via the insulating oil 5: Then, as shown in an equivalent circuit in FIG. 3, the bridge constituted by resistors R.sub.G1, R.sub.G2, R.sub.G3, and R.sub.G4 of the distortion gages produces a difference between a vacuum and a space in the recessed portion of the chip 1, i.e., an output corresponding to an absolute pressure. This output is converted to a sensor signal having a predetermined characteristic and is outputted from a V.sub.out terminal. Accordingly, it is necessary to lead out four terminals, namely, a terminal 91 for applying a supply voltage V.sub.cc through the container 3, a ground terminal 92 for connecting the chip to Gnd, and two terminals 93 and 94 connected to the signal processing circuit 44.
FIG. 4 is another example of the conventional art, and portions which are common to those of FIG. 2 are denoted by the same reference numerals. In this case, the pressure to be measured is applied to an etched-side surface of the diaphragm. The silicon chip 1 is secured on the pedestal 2 where a hole 11 for introducing the pressure to a central portion is located. In addition, the pedestal 2 is secured on a substrate 12 coupled with the container 3. A hole 13 for introducing the pressure to an extension of the hole 11 in the pedestal 2 is provided in this substrate 12 as well. The insulating oil 5 is sealed in a space surrounded by the silicon chip 1, the substrate 12, the cylindrical container 3, and the sealed diaphragm 4 whose periphery is airtightly coupled with the container 3. The pressure to be measured is introduced through the hole 7 formed in the cover 6, and is transmitted to the etched surface of the diaphragm of the silicon chip 1 via the insulating oil 5. An unetched surface of the silicon chip 1 is covered with a cover 14, and is open to the atmosphere through a hole 15 formed in the cover 14. As a result, this sensor operates as a relative pressure sensor. In the same as the example of the conventional art shown in FIG. 2, the circuit substrate 40 for outputting a predetermined sensor signal by performing signal processing as well as the molded resin 43 for protection are attached to the outside of the container 3.
To disseminate the above-described double diaphragm-type semiconductor pressure sensors, it is necessary to make the sensors compact in size and effect a cost reduction. For the compact size and reduced cost, a reduction of the number of components parts used is effective.